For many years drilling rigs have utilized the concentric pipe method of drilling wherein a fluid, such as air or a mixture of compressible and incompressible fluids, is forced down the annulus between an inner and outer drill pipe to a drill bit located at the lower end of the concentric string, whereupon cuttings formed by the bit are forced to travel back uphole through the inner pipe, and to the surface of the ground.
From time to time, while making hole, it is necessary to add or subtract another joint of inner and outer pipe respective to the concentric drill string and when this happens, all of the liquids and solids contained within the inner pipe and the annulus between the pipes gravitate back down toward the bit, seeking equilibrium of the hydrostatic head in the two columns of fluid. The cuttings congregate and coagulate at the bottom of the borehole and usually the weight of the fluid and cuttings in the inner pipe exceeds the pressure available in the annulus between the inner and outer tube, whereupon, the fluid and cuttings will migrate into the annulus. Often, this mixture will have a consistency between mud and wet concrete and when an extended time delay in restoring the drilling air supply occurs, it is possible for the system to become clogged. It would therefore be desirable to provide an automatic check valve device in the annulus between the inner and the outer tubing whereby flow downhole through the annulus toward the bit occurs with little pressure drop across the valve device while reverse flow is precluded. This desirable feature enables the inner tubing to remain loaded with fluid rather than seeking equilibrium with the fluid in the annulus.
This check valve device therefore blocks the migration of fluid from the axial passageway into the annulus of the concentric pipe and simultaneously reduces the time required to repressure the drilling pipe and restore the drilling operation. Accordingly, the check valve sub can be installed in the concentric string of drill pipe where it serves as a one way fluid or air trap.
Therefore, the check valve sub of this invention can be located in series relationship with respect to the concentric drilling pipe. Preferably the check valve sub is located near the bit to reduce the volume of compressible gases that may be trapped downhole of the check valve within the annulus.
The provision of an improved check valve sub that achieves the above goals is the subject of this invention.